CMOS has been the basic logic building block in the digital-dominant world for decades. Device dimensions have been continuously shrunk in order to achieve higher performance as well as higher packing density. Technology advancement demands more and more functions to be integrated on a single chip, and thus one MOS design cannot fulfill all requirements in an integrated circuit. For example, in a traditional circuit that employs micron technology or earlier technologies, the operation voltage of the core circuit was typically in a range of about 2.5V to 3.3V. However, as the integrated circuit size continues to shrink, the core operation voltage is lowered. When the deep sub micron technology is employed, the size of a circuit is further reduced to about 0.25, 0.18 or even 0.13 micron, and the operation voltage drops to around 1V. It is expected that the core operation voltage will continue to fall when the integrated circuit size continues to shrink. While the core operation voltage falls, the operation voltage of the I/O circuit often stays at a higher value. As a result, the MOS devices in the I/O circuit and the core circuit are expected to work under different operation voltages.
Scaling of the integrated circuit also causes other problems. In traditional IC processes, gate electrodes are typically formed of polysilicon. One of the reasons for polysilicon's wide use is that the work function of polysilicon gate electrodes can be changed easily by doping with different impurities. However, polysilicon has depletion problems, so metal gate electrodes were introduced, particularly for device formation in core regions, to avoid the poly depletion phenomenon.
Previously discussed issues demand customized manufacturing processes. U.S. Pat. No. 6,432,776 provides an integrated circuit having a core section and an I/O section. A device in the core section includes a metal gate and a thin oxide gate dielectric, while a device in the I/O section has a polysilicon gate and a thick oxide gate dielectric. U.S. Pat. No. 6,777,761 also discloses an integrated circuit having two regions. In a first region, a device comprises a pure metal gate and an oxide gate dielectric. In a second region, a device comprises a polysilicon gate and an oxide gate dielectric.
By using these prior art solutions, one part of a chip can have a device with a reliable, high-yield polysilicon gate, while the other part of the chip can have a high-performance MOS device with a metal gate. These integrated circuits and manufacturing processes, however, have a potential problem. Typically, metal gates are thinner than polysilicon gates to alleviate the difficulties associated with the metal etching process and to improve the gate stack profile control, thus there is a step height between the top surfaces of the metal gates and the polysilicon gates. Additionally, gate dielectrics in the core region are typically thinner than gate dielectrics in the I/O region, further increasing the step height. This step height adds complexity and cost for subsequent manufacturing processes.